Electrohydraulic actuators are well-known in the art, and comprise a motor, a hydraulic pump and a hydraulic piston/ram. In operation, the motor is powered to drive the pump, which is used to pump hydraulic fluid to actuate the piston. An EHA may be used on an aircraft, with the piston operatively connected to a flight control surface.
A typical motor for an EHA typically comprises a magnetic rotor mounted to a rotary shaft, and a stator surrounding the rotor. The stator consists of a magnetically permeable core and field windings at locations on the core. The field windings may also be potted (i.e. coated and encased in a solid polymer compound, e.g. thermosetting plastics or silicone rubber) for resistance to shock, vibration, internal pressure and for exclusion of moisture and any corrosive substances. This is in contrast to the more widely used process of impregnating field windings with thin varnish. Supplying electrical current to the field windings generates a magnetic field in the stator core, which interacts with the magnetic rotor, and causes the rotary shaft to rotate. The rotary shaft can then be used to drive the pump, as is known in the art.
EHAs may be self-contained, in that the motor and pump are also used for holding the hydraulic fluid, such that the need for separate reservoirs and fluid connectors is removed. This may simplify actuator construction and make the EHA more reliable than conventional hydraulic actuators. An example of such an EHA is disclosed in U.S. Pat. No. 4,529,362.
A problem exists for motors used in EHAs of the art, in that segregating the motor from the hydraulic fluid in the pump is very difficult. All hydraulic fluids will attack and degrade certain coatings, plastics and elastomers. One such fluid, Skydrol®, a phosphate ester based fluid, is particularly well known for degrading a wide range of paints, polymers and elastomers. Contamination of the field windings, wiring and insulation system with such fluids may cause the electrical failure of the motor. The present disclosure seeks to provide an improved motor topology to address this problem.